Wavelength converting materials, such as phosphors, are used for tuning the color of light emitting diode (LED) based light sources. Phosphors in combination with blue LEDs are used to produce white light. Depending on the type of phosphors and the amount of conversion, the color can be tuned to achieve a desired color such as cool white or warm white. The white light is produced by a combination of transmitted (unconverted) blue light and converted, often yellowish, light.
When the phosphor is arranged in a substrate or layer separate, i.e. at a certain distance, from the LED, it is referred to as a remote phosphor layer. Such a remote phosphor layer may be provided directly in an outer envelope of the lighting device or as a separate layer inside the envelope. Examples of such lighting devices are shown in CN201606695 and EP2293355.
A problem with remote phosphor layers is that the color distribution of light emitted from the exit surface, i.e. the surface of the remote phosphor layer from which light is emitted, may be non-uniform. This is in particular the case in LED-based tube lamps having e.g. blue LEDs and a phosphor mixture in the curved envelope, wherein yellow lines are visible at the edges of the envelope at angles close to ±90° with respect to an optical axis of the lamp.